Hydrocarbons are cracked into light fractions by contacting them under elevated temperature conditions with a cracking catalyst. During this cracking process, a certain amount of coke is deposited on the cracking catalyst particles. This coke buildup reduces the activity of the catalyst. The spent catalyst is regenerated by removing the hydrocarbons from the spent catalyst and contacting it under elevated temperature conditions with a free oxygen-containing gas, which will be called air in the following for simplicity. This gas can, however, be any free oxygen-containing gas. By this procedure the coke and the air react so that coke is burned off from the catalyst particles. The thus regenerated catalyst particles are returned to the cracking step.
The control of the regenerator is a very difficult problem since the combustion of the coke is influenced by several process parameters, some of which are uncontrollable. Typically, the catalyst regenerators are controlled by controlling the flow of air into the regenerator responsive to a temperature measurement within the regenerator.
Several difficulties occur in such temperature control systems due to the fact that the air sources are usually turbocompressors, that the air flow rate has to be changed often and accurately for good temperature control, which is difficult if not impossible with the turbocompressors, and that the air flow signals are usually very noisy. It would, therefore, be desirable to have a more accurate system available for controlling the air flow to a regenerator, as blowers such as turbocompressors are being used as an air source because of their higher efficiency.